make assumption output in trace reachable

Assumptions are never violated, instead they restrict the search space for non-det values

Author: polgreen

src/goto-programs/goto_trace.cpp

@@ -278,7 +278,7 @@ void show_goto_trace(
       break;
 
     case goto_trace_stept::typet::ASSUME:
-      if(!step.cond_value)
+      if(step.cond_value)
       {
         out << "\n";
         out << "Assumption:" << "\n";